The present invention relates to a Cellular telecommunications network and in particular to a method of positioning a Mobile Station (MS) communicating with such a network. The present invention is especially applicable to a Cellular telecommunications network operating in accordance with a Code Division Multiple Access (CDMA) protocol. In some such systems all Base Transceiver Stations (BTS's) transmit to all MS's within the same portion of the radio frequency spectrum which is referred to as the downlink, whilst all MS's transmit in a different portion of the radio frequency spectrum which is referred to as the uplink.
There is a requirement within such systems, to enable MS's operating within the network to be positioned (i.e. for the position of such MS's to be determined) by the network. A recent proposal to achieve this aim involves a Mobile Station attempting to detect signals transmitted by at least two geographically separated Base Stations in addition to the main Base Station with which it is communicating. The MS can then report the time of arrival of the signals received from the at least two other Base Stations to the network via its serving Base Station g. Provided the network has knowledge of the time at which the signals left the respective other Base Stations, it is able to calculate the time taken for the signals to travel from the other Base Stations to the Mobile Station and from that calculate the distance of the MS to each of these base stations. A round trip delay measurement enables calculation of the MS's distance to the serving BTS. Information about the MS's distance to at least 3 BTSs for which the exact position is known allows the network to perform triangulation and evaluate the MS's position.
However, there is a problem with implementing this method in a CDMA system since all Base Stations transmit within a common frequency range (the downlink, also sometimes referred to as the forward link). In order for signals on a particular channel to be detected they have to be received at power that has a certain ratio to the overall power of all other signals transmitted in the same spectrum portion. Channels transmitted from each BTS are transmitted with adequate power for reception only within the cell's area in order to avoid excess interference to other cells. The power at which any individual channel from any BTS are received in other cells is low and it usually does not satisfy a power ratio requirement that would satisfy the requirements for a time of arrival measurement. This is especially true for MSs that are near their serving BTS where the power from this BTS—which is seen as interference for measurements on signals from other BTSs—is very high. In order to solve this problem, it has been proposed that when a Mobile Station is to be positioned, the main Base Station with which that mobile is communicating will cease all transmissions in the downlink for short periods at predetermined times to enable the Mobile Station to detect the transmissions from other Base Stations during these times.
There are several drawbacks with this scheme, including the following three principal drawbacks. Firstly, during each quiet period (often referred to as an idle slot) the main Base Station is unable to transmit any information to any of the Mobile Stations with which it is in communication. Secondly, since the Base Stations are not normally synchronous with one another, some mechanism must be provided for enabling all of the Base Stations to know about the relative timing of all neighbouring Base Stations. This adds additional complexity to the network. Thirdly, the Mobile Station requires a fairly high specification Automatic Gain Control unit (AGC) as part of its receiving circuitry, to be able to quickly switch from detecting the transmissions from its main Base Station to detecting the much lower power transmissions from other more distant Base Stations.
An alternative known method of positioning of mobile stations within a CDMA cellular telecommunications network involves the use of beacons which are strategically located throughout the network. The beacons simply transmit signals at known power level which are able to be detected by the mobile stations within the network which are sufficiently close to them. Included within the signals transmitted by the beacons is information identifying the beacons. A mobile station which desires to be positioned detects the signals and measures the strength of the received signals. From the strength of the received signals and the known transmitted power the path loss is evaluated and from that the distance that would correspond to such a loss in that particular environment. Information on the distance of the MS to at least 3 geographical locations (beacon locations) allows calculation of the MS's position. The drawbacks associated with this method are that measuring received signal strength is not very reliable, the path loss is environment dependent and the dependency may not be very accurate, and the beacons transmit on the downlink inducing some capacity loss.